The herbicidal activity of 2-(4-aryloxyphenoxy)propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Patent No. 4,531,969 discloses that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are required to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages.
To exploit the agronomic benefits of these advantages, it is necessary to efficiently resolve racemic mixtures of herbicides that are normally produced industrially. Various methods for obtaining high concentrations of individual enantiomers are known. For example, individual enantiomers can be obtained by direct synthesis employing appropriate optically active starting materials. Alternatively, a racemate can be resolved by conversion with an optically active reagent into a mixture of diastereomers which can subsequently be separated on the basis of their different physical properties. Theoretically, one can obtain essentially 100 percent of the desired enantiomers by these methods. In practice, however, the products typically contain from 70 to 95 percent of the desired enantiomer and, correspondingly, from 5 to 30 percent of the other optical isomer. Such products are then said to possess an optical purity of 40 to 90 percent, i.e., from 40 to 90 percent of the mixture is the desired enantiomer and from 10 to 60 percent is a racemic mixture.
An efficient technique for the continued enrichment of mixtures already containing an excess of a particular enantiomer is resolution by entrainment, otherwise known as preferential crystallization. Unlike other resolution methods, entrainment does not require an auxiliary chiral compound such as, for example, an optically active reagent to form diastereomers or an optically active solvent, in order to achieve an effective resolution. The principles of enantiomer resolution by direct crystallization, particularly by entrainment, have been reviewed by J. Jacques et al. in Enantiomers, Racemates, and Resolutions, John Wiley & Sons, Inc., New York, N.Y., 1980, Chapter 4, pages 223-241.